The tropics in trouble and some hope

Disappearing habitat

Researchers say that it’s reasonable to expect huge losses in tropical insect abundance and diversity for two reasons: forest destruction and climate change. Other threats — pesticides and invasive species — are also mentioned, but less frequently and emphatically.

“Because the climate is warming in the tropics, and tropical forests are experiencing the greatest deforestation rates, it’s probable that insects will not fare well,” notes Danielle Salcido, a University of Nevada graduate student studying Neotropical insects.

Habitat loss is the first concern for most entomologists questioned, and that’s hardly surprising: roughly a quarter of the Amazon rainforest, the world’s largest and most diverse, is gone. The Congo, the globe’s second largest rainforest, saw an area erased nearly the size of Florida from 2000-2014. And Borneo, the Earth’s third largest island, has lost half its forest cover in just 60 years.

In fact, nowhere have forests been falling faster in recent decades than in Southeast Asia, where Indonesia is now the leader of global forest destruction, even beating Brazil. “We can be confident that populations of many [insect] species in Southeast Asia will be doing poorly overall,” says Lewis, despite sparse data. He adds that forest conversion to agribusiness “especially oil palm plantations” means that “forest-dependent insect species will have crashed.”

Some species may see rapidly rising populations — those that do well in disturbed environments — but others will suffer. “A few winners, but many losers,” Lewis concludes.

African entomologists likewise cite habitat destruction as the most pressing issue. University of Pretoria entomologist Mervyn Mansel reports that the insect species that he studies, lacewings — many endemic to South Africa — are imperiled by habitat loss. Still, he adds, it’s impossible to extrapolate onto other insect families.

“This [problem] is extremely difficult to assess on a general scale. Some insect groups may be disappearing due to excessive urbanization [and] habitat destruction… On the other hand, some insect groups are flourishing due to favorable conditions created by humans,” he says.

Mpala Research Center Executive Director and entomologist Dr. Dino Martins, says he’s anecdotally noticed a decline in insects in his home country of Kenya, but there’s no hard data yet. Still, he thinks habitat loss is the biggest factor at work in East Africa — especially due to the loss of particular plants: “If you lose endemic plants, it’s obvious that the endemic insect species that live on them, pollinate them, and parasitize them, would disappear.”

Still, not all tropical nations may be seeing uniform rates of decline. Dr. Vojtech Novotny, a University of South Bohemia entomologist who studies insects in Papua New Guinea, says he hasn’t noticed losses. “If I were to speculate, I would not expect a serious decline taking place here. I have not noticed it over the past 25 years.” This may be because, unlike many tropical countries, Papua New Guinea still has significant primary forest cover. Though that may be changing, as the nation undergoes an epidemic of illegal logging.

“Insect species and population densities have been gradually declining in the Neotropics for at least a century, anywhere that humans have begun their usual overpopulation, clearing of natural ecosystems for human purposes, and massive uses of pesticides,” Janzen explains. He adds that climate change is now an “impact that blankets over it all, rather than being in pockets and regions.”

Climate change: the heat is on

While most entomologists blame habitat loss as being key to tropical insect declines to date, they see global warming as an up and coming ecosystem-wide exterminator.

“Climate change is a more insidious change which could cause just as many extinctions in the long term,” Lewis says. And it could hit insects in multiple ways: via intensifying extreme weather — more and longer heat waves and sudden deluges. At the same time, rainforests and cloud forests that once saw near-constant moisture, are drying out, due to the one-two punch of escalating heat and longer droughts, something Janzen and Hallwachs described in their study sites.

“It’s important to remember that small increases in temperature have large negative effects on tropical insects,” notes Salcido.

In Kenya, Martins expresses another concern: that climate change could knock insects and plants “out of sync.”

“We are disturbing the [climate] system to the point where the productivity and the interactions between species are being impacted,” he says. Flowers bloom early or late, and there’s no one there to pollinate them, or insects emerge looking for their food sources, but flowers have yet to bloom.

Because we know so little about tropical insect lifecycles, we know even less about how families or species might respond to climate change. But some research suggests the news won’t be good: a 2018 study found that heatwaves created under laboratory conditions (5-7 degrees Celsius above optimal temperatures for five days) decimated the sperm of male flour beetles. The authors even found that “successive heatwaves almost sterilize males.” If this study carries over widely to the field, climate change may turn whole populations of male insects into eunuchs.